Mount Thielsen Wilderness, Protected wilderness area in southern Cascade Range, Oregon, United States.

Mount Thielsen Wilderness covers 55,100 acres (22,300 ha) around Mount Thielsen in the southern Cascade Range, within the Deschutes, Umpqua and Fremont–Winema national forests. Lodgepole pine grows in the lower portion, giving way to mountain hemlock and fir up to the timberline at about 7,200 feet (2,200 m). The terrain ranges from 5,000 feet (1,500 m) to the 9,182-foot (2,799 m) summit, with flat and moderately rolling hills turning into steep, sharply dissected ridges toward the Cascade crest.

Congress designated this area as a protected wilderness in 1984 under the National Wilderness Preservation System. The decision came as part of a broader effort to safeguard undeveloped mountain regions throughout the western United States.

The wilderness holds Maidu Lake, whose outlet forms the headwaters of the Wild and Scenic North Umpqua River, and Lake Lucille; both are named among the area's popular lakes. The Pacific Crest Trail crosses the area for 26 miles, and the wilderness lies within the Deschutes, Umpqua and Fremont–Winema national forests.

This area is best explored on foot, with extensive trail networks ranging from easy walks to challenging mountain hikes. Summer provides the most favorable conditions with clear paths and stable weather for safe outdoor travel.

The sharp peak of Mount Thielsen receives so many lightning strikes due to its exposed position that locals call it the Lightning Rod of the Cascades. This remarkable feature makes the summit one place to definitely avoid during thunderstorms.